In the current research, locally produced steel slag in Egypt was used as full a replacement of the natural aggregate for producing pressurized underwater concrete. The experimental program consists of casting and testing four groups with a total number of twenty underwater-concrete mixes. Concrete mixtures contain anti-washout admixtures (0.2, 0.3, 0.4 and 0.5%) by weight of cement with cement contents (400,450,500 and 550 kg/m3) for water-cementations materials ratio, which was set at (0.5, 0.444, 0.4 and 0.364%) respectively. All concrete mixtures contain silica fume and high-range water reducing (15and 4%) respectively by the weight of cement. The fine to steel slag coarse aggregate were 1:1. The concrete mixtures were tested for slump, slump flow, compressive strength and washout resistance by using two test methods based on different principles. The first method is the plunge test CRDC61, widely used in North America, and the second method is the pressurized air tube, which has been manufactured for this research and developed in order to trigger the effect of water pressure on washout resistance of underwater mixtures. The compressive strength of test results was compared to concrete cast under water with that cast in air. Test results indicated that the use of AWA facilitates the production of UWC mixtures with the additional benefit of lower washout resistance. The new technique of simulating pressurized UWC is reliable for detecting UWC properties. Adding AWA with (0.3-0.5%) by weight of cement makes all mixtures acceptable according to Japanese Society of Civil Engineers.